The frequency response of a interconnect (e.g. a PCB/cable) used for data transmission attenuates the high frequency components of a data stream. The level of this attenuation is illustrated schematically in FIG. 1, showing that it arises at above a frequency labelled ω2. When the attenuation starts below the maximum frequency of the data stream, this leads to a phenomenon known as inter-symbol interference (ISI), in which certain data patterns cause thinning of the data pulse widths, and hence there is data dependant jitter. A worst case example of this is for a data pattern in which the transmitted data has a single data high amongst data lows, as shown in FIG. 2(a). The attenuation of the high frequency components of the single data high results in a pulse which has reduced amplitude and long rise/fall times. Consequently pulse widths are reduced as shown in FIG. 2(b).
The CML type driver, illustrated in FIG. 3, includes a differential pair at the transmitter and loads at the receiver. The two inputs (which are equal and opposite voltage signals) are amplified by an amplifier 1, and then fed as equal and opposite signals txn and txp to respective transistors 3,5. The transmission lines 7, 9 have an impedance Z0. The loads R are at the termination for the transmission lines. An rx amp 10 is provided at the output. The gain of the driver is proportional to the width of the transistors 3,5 in the differential pair. Wider transistors provide more gain, A CML-type driver illustrated in FIG. 3 provides equal gain at all frequencies.
However, drivers are known which provide a pre-emphasis which flattens the frequency response of the interconnect at higher frequencies, preferably up to and beyond the data signal frequency. This is illustrated in FIG. 4, in which FIG. 4(a) is the frequency response of the interconnect, FIG. 4(b) is the frequency response of the pre-emphasis filter which has a pole at ω2. and FIG. 4(c) is the frequency response of the combination of the two, which is flat up to frequency ω1. The pre-emphasis filter has a zero at ω2 to cancel the attenuation of the interconnect. The pre-emphasis filter also has a pole at ω1 but this is higher than the maximum data frequency. Such known drivers employ passive components to achieve the frequency response shown in FIG. 4(b). This means that, taking the pre-emphasis and data transmission period as a whole, low and high frequency components are both attenuated by the same amount, and little or no thinning of the pulse widths occurs.